1. Field of the Invention
The present invention relates to an image processing apparatus, an image forming apparatus, and a computer product.
2. Description of the Related Art
In recent years, with the improvement of an image processing technology and an image forming technology, banknotes, marketable securities, or the like are copied so faithfully using a digital color copying machine or a digital color multifunction device, that the genuine banknote, security, or the like cannot be easily distinguished from a copy thereof. Therefore, it is necessary to take measures to make it totally impossible to copy or make it impossible to correctly copy special documents such as the banknote and the security.
For example, even in case of general company documents other than the special documents such as the banknotes and the security, there are a large number of confidential documents that are prohibited from being outputted by means of copying and the like. Thus, it is necessary to take measures to make it totally impossible to copy or make it impossible to correctly copy such confidential documents also.
Under such circumstances, conventionally, various inventions have been made to restrain copying of documents such as the special documents and the confidential documents that are prohibited from being reproduced. Specific examples of such inventions are described below.
There are various methods of distinguishing the special documents such as the banknotes and the securities. Japanese Patent Application Laid-Open No. H6-125459 and Japanese Patent Application Laid-Open No. 2001-86330 disclose a method of comparing input image data of a document and a specific mark (pattern data) registered in advance according to a pattern matching method, and if the specific mark is present in the image data, judging that the document is a special document is proposed. Upon judging that the document is the special document, it is easy to prohibit copying of the document, that is, prevent copying the document.
As an example of a technology for distinguishing the confidential documents prohibited from being copied, Japanese Patent Application Laid-Open No. H7-36317 and Japanese Patent Application Laid-Open No. H7-87309 disclose inventions for detecting a mark, which is attached to a confidential document for indicating the confidentiality of the document. The inventions make use of the fact that, in general, the mark including a confidentiality seal, a copy prohibit mark, or the like is put on the confidential documents prohibited to be copied. When it is judged that a document is a confidential document, it is easy to prevent the document from being copied.
Japanese Patent Application Laid-Open No. H9-164739 and Japanese Patent Application Laid-Open No. 2001-197297 disclose techniques for controlling copying by embedding a base pattern in a document image desired to be prohibited from being copied. In the techniques, a sheet having a base pattern including a base area and a message area formed thereon as a background is used as a sheet for a document image. The base pattern is not so conspicuous in the document image, and does not hinder reading or the like of information included in the document image. However, when the document image embedded with such a base pattern is copied, for example, a pattern of the message area emerges. Thus, for example, if characters “copy prohibited” are given as the pattern of the message area, it is quite obvious that a copy of the document is a confidential document prohibited from being copied. Consequently, it is possible to psychologically restrain a person from copying the document.
Another technique of controlling copying by embedding a base pattern in a document image desired to be prohibited from being copied, is a technique of applying document distinction disabling processing for painting out a document image with solid when the document image is copied.
However, in this technique, there can be problems as described below.
In the document distinction disabling processing for painting out a document image with solid when the document image is copied, only a fixed color is used as a color material necessary for the solid color. In other words, a degree of decrease in the color material used for the document distinction disabling processing is large compared with those of other color materials. Therefore, when the color material used for the document distinction disabling processing frequently used, general use of the color material is hindered.